Wwise is described as '"Wwise® is the most advanced, feature-rich interactive sound engine for games, bar none. Whether you're an indie or a multi-million dollar production, Wwise will work for you." - Audiokinetic'. There are five alternatives to Wwise for Windows, Linux, Mac and Self-Hosted solutions. The best alternative is PortAudio, which is both free and Open Source. Other great apps like Wwise are RtAudio (Free, Open Source), FMOD Ex (Free Personal), OpenAL Soft (Free, Open Source) and SoLoud (Free, Open Source).
RtAudio is a set of C++ classes that provide a common API for realtime audio input/output across Linux, Macintosh OS-X and Windows operating systems. RtAudio significantly simplifies the process of interacting with computer audio hardware.
OpenAL Soft is an LGPL-licensed, cross-platform, software implementation of the OpenAL 3D audio API. It's forked from the open-sourced Windows version available originally from the SVN repository at openal.org.